Unsteady Ground has a shorter casting time and less energy cost than Earthquake or Churning Earth. Used with Aftershock, it can do a large amount of damage to attacking melee fighters in a short amount of time.
This is one of a small number of DoTAoE spells that is likely to deal its full damage against the AI without other snares. The knockdown, especially if paired with a skill like Earthbind, will still allow the AI to attack for a split second after getting up, before trying to run away. This will cause them to fall down again.